1. Field of the Invention
This invention relates to seismic exploration in general. More specifically, it concerns an improved method of reflection-type seismic exploration that is particularly applicable to marine operations.
2. Description of the Prior Art
It has been known for quite some time that in marine seismic operations, there are particular problems that are not encountered in land operations. One aspect stems from the fact that in marine operations the seismic detectors placed below the surface of the water are sensitive to seismic waves in the water regardless of their direction of travel. Furthermore, pressure-type detectors are usually used, whereas in land operations the detectors are ordinarily a displacement or inertia type.
Heretofore, in marine operations, the seismic waves were often generated by detonation of an explosive charge which was usually placed at a depth of ten feet or less below the surface. This avoided interference produced by the phenomenon commonly called "bubble bounce", which interference is generated by charges fired at greater depth. However, a problem has remained in offshore seismic exploration which stems from the fact that water is an efficient propagator of sound and seismic energy waves travel long distances horizontally. Therefore, any such signals returning to or otherwise arriving at the detectors or hydrophones, are interfering and so detrimental to the desired reflection data.
While it has heretofore been suggested to use a plurality of vertically displaced detector spreads in seismic prospecting carried out in water, such suggestion dealt with overcoming interferences from energy reflected back down into the water by air-water interface. No part of that concept was related to horizontally travelling seismic energy.
Consequently, it is an object of this invention to provide an improved method of marine-type seismic surveying. Furthermore, another object is to provide a method of marine-type seismic exploration which eliminates undesirable horizontally travelling seismic wave energies from a recording.